On Tuesday, she posted a photo of her backside as she looked in a make-up mirror with the caption: "...Secret Project."

Gomez, 21, looks all grown up in a sheer black costume.

Her on/again, off/again boyfriend Justin Bieber has also been posting provocative photos on the photo sharing website. Earlier this week, the pop star emulated acting icon James Dean in a black and white photo.

The pair reportedly can't stay away from each other.

A source recently told People magazine: "He's very charming. I'm not sure what's going on with him right now, but he's very smart, and used to be a very fun and funny kid. Whenever they're not talking, he has to do something to get her attention, like posting [a photo of her at the Vanity Fair Oscars party], captioned 'most elegant princess.'"

Photo Credits: Instagram

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